In the completion of oil and gas wells it is standard practice to set or cement at least one string of casing within the well bore. Casing strings are cemented in the well bore to prevent fluids from migrating from the production zone through the annulus between the casing string and the well bore to the surface or other zones where for example fresh water may be contaminated. In addition, there are regulations which require that some zones be cemented off.
In cementing a casing string, a cement slurry is pumped down the interior of the casing string, out the lower end, into the annulus between the string and the well bore. However, to effect an efficient cementing job, the complete annulus needs to be cemented without pockets in the cement and without areas in which the string is contacting the wall of the well bore. To facilitate obtaining an effective cementing job the casing is commonly centered in the well bore with centralizers which are disposed about the casing string. In addition, the centralizers aide in running the pipe into the hole without hanging up.
Centralizers may also be used on casing or pipe strings, such as tubing, which are hung within another string of casing or pipe. These inner strings may be cemented within the outer pipe string or they may not be.
Centralizers for casing, tubing or pipe commonly are constructed of a low carbon steel having a tubular body or sleeve adapted to fit around a pipe joint. These prior art centralizers usually include outwardly bowed springs having opposing ends connected to opposite ends of the sleeve. Although the resiliency of the bow strings enables them to move through tight spots in the well bore, they may not support the weight of the casing string, especially in a highly deviated well bore.
In another type prior art centralizer, the bow strings are replaced by solid strips of metal which are tapered at each end to provide outer spaced bearing surfaces for engaging the well bore or the outer casing. Although less prone to collapse than bow springs under the weight of the casing string, these metal strips are often not strong enough to prevent bending upon contacting an obstruction or turn in the well bore. As a result, the centralizer and the casing may become wedged in the well, and, in any case become unsuitable for providing a suitable cementing job.
These prior art metal centralizers have further drawbacks, especially, when run and set within another string of pipe. One of the drawbacks is when the metal centralizer is run into or when it vibrates due to slugs in production it contacts the outer pipe string and may cause a spark, which can be very hazardous in the hydrocarbon filled well. Also, these metal centralizers create a corrosion problem with the casing strings which it contacts through electrolysis. These metal centralizers also are susceptible to damage when running acid and circulating the acid back out of the hole. Additionally, there is a concern with scrapping the inner diameter of stainless steel tubing when running stainless/duplex stainless steel tubing having metal centralizers.
It would be a benefit, therefor, to have a centralizer adapted to fit about a string of pipe for centering the pipe in a well bore or within an outer string of pipe which is constructed of a non-metallic material. It would be further benefit to have a centralizer which has non-sparking properties. It would be a still further benefit to have a centralizers that provides cathodic protecting between strings of casing. It would be an additional benefit to have a centralizer which is resistant to deterioration due to acid. It would be a still further benefit to have a non-metallic centralizer which is lightweight while having sufficient strength to withstand the forces encountered in casing operations.